However if a web page is loaded via the socks proxy and a browser in between establishing forwarding and attempting to stop any further multiplexing requests, then the 'stop' command always works without any problem.

Otherwise the ssh process mostly exits, which from what I can gather would be correct if there were no open connections remaining (assuming 'ControlPersist' is not set to 'yes'), but not if as in this case ssh is forwarding (i.e., an open connection still exists).